The invention relates to receivers for broadcast digital television signals and, more particularly, to filtering for the cancellation of multipath distortion in the received signals, which filtering is adaptive responsive to training signals inserted into the broadcast digital television signals.
The Advanced Television Systems Committee (ATSC) published its A/53 Standard for Digital Television Broadcasting in 1995; and that standard is referred to simply as xe2x80x9cA/53xe2x80x9d in the rest of this specification. In 1995 ATSC also published its A/54 Guide to the Use of the ATSC Digital Television Standard, which guide is referred to simply as xe2x80x9cA/54xe2x80x9d in the rest of this specification.
The broadcast digital television signal to which the receiver synchronizes its operations is called the principal signal, and the principal signal is usually the direct signal received over the shortest transmission path. The multipath signals received over other paths are thus usually delayed with respect to the principal signal and appear as lagging ghost signals. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes its operations to a (longer path) signal that is delayed respective to the direct signal, there will be a leading ghost signal caused by the direct signal, or there will a plurality of leading ghost signals caused by the direct signal and other reflected signals of lesser delay than the signal to which the receiver synchronizes. While the term xe2x80x9cghostxe2x80x9d was usually used by workers in the analog television art to refer to a multipath signal component other than the principal signal, many workers in the digital television art customarily refer to the multipath signal component using the term xe2x80x9cecho signalxe2x80x9d or the shorter term xe2x80x9cechoxe2x80x9d because of its similarity to a reflection on a transmission line. The leading ghost signals are referred to as xe2x80x9cpre-ghostsxe2x80x9d or xe2x80x9cpre-echoesxe2x80x9d, and the lagging ghost signals are referred to as xe2x80x9cpost-ghostsxe2x80x9d or xe2x80x9cpost-echoesxe2x80x9d. The ghost or echo signals vary in number, amplitude and delay time from location to location and from channel to channel at a given location. On Jan. 19, 2000, A. L. R. Limberg filed U.S. provisional application Ser. No. 60/177,080 titled xe2x80x9cGHOST CANCELLATION REFERENCE SIGNALS FOR BROADCAST DIGITAL TELEVISION SIGNAL RECEIVERS AND RECEIVERS FOR UTILIZING THEMxe2x80x9d, which application is incorporated herein by reference and is referred to simply by its serial number in following portions of this specification. At the time 60/177,080 was filed, it was generally assumed that post-ghosts with significant energy are seldom delayed more than forty microseconds from the reference signal and that pre-ghosts with significant energy seldom precede the reference signal more than three to four microseconds.
Ghost signals that are displaced in time from the principal signal substantially less than a symbol epoch, so as to affect channel frequency response, but not enough to overlap symbols with ghosts of symbols more than a symbol epoch away are sometimes referred to as xe2x80x9cmicroghostsxe2x80x9d. These short-delay or close-in microghosts are most commonly caused by unterminated or incorrectly terminated radio frequency transmission lines such as antenna lead-ins or cable television drop cables. Ghost signals that are displaced in time from the principal signal by most of a symbol epoch or by more than one symbol epoch are sometimes referred to as xe2x80x9cmacroghostsxe2x80x9d to distinguish them from xe2x80x9cmicroghostsxe2x80x9d.
The transmission of the digital television (DTV) signal to the receiver is considered to be through a transmission channel that has the characteristics of a sampled-data time-domain filter that provides weighted summation of variously delayed responses to the transmitted signal. In the DTV signal receiver the received signal is passed through channel-equalization and ghost-suppression filtering that compensates at least partially for the time-domain filtering effects that originate in the transmission channel. This channel-equalization and ghost-suppression filtering is customarily sampled-data filtering that is performed in the digital domain. Time-domain filtering effects differ for the channels through which broadcast digital television signals are received from various transmitters. Furthermore, time-domain filtering effects change over time for the broadcast digital television signals received from each particular transmitter. Changes referred to as xe2x80x9cdynamic multipathxe2x80x9d are introduced while receiving from a single transmitter when the lengths of reflective transmission paths change, owing to the reflections being from moving objects. Accordingly, adaptive filtering procedures are required for adjusting the weighting coefficients of the sampled-data filtering that provides ghost-cancellation and equalization.
Determination of the weighting coefficients of the sampled-data filtering that provides channel equalization and ghost suppression is customarily attempted using a method that relies on analysis of the effects of ghosting on all portions of the transmitted signal or using a method that relies on analysis of the effects of ghosting on a training signal or ghost-cancellation reference (GCR) signal included in the transmitted signal specifically to facilitate such analysis. While the data field synchronizing (DFS) signals in the initial data segments of the data fields in the DTV signal specified by A/53 were originally proposed for use as a training signal sequence, they are not well-designed for such purpose. So, most DTV manufacturers have attempted to use decision-feedback methods that rely on analysis of the effects of ghosting on all portions of the transmitted signal for adapting the weighting coefficients of the sampled-data filtering. Decision-feedback methods that utilize least-mean-squares (LMS) method or block LMS method can be implemented in an integrated circuit of reasonable size. These decision-feedback methods provide for tracking dynamic multipath conditions reasonably well after the channel-equalization and ghost-suppression filtering has initially been converged to substantially optimal response, providing that the sampling rate through the filtering is appreciably higher than symbol rate, and providing that the rates of change of the dynamic multipath do not exceed the stewing rate of the decision-feedback loop. However, these decision-feedback methods tend to be unacceptably slow in converging the channel-equalization and ghost-suppression filtering to nearly optimal response when initially receiving a ghosted DTV signal. Worse yet, convergence is too slow when tracking of dynamic multipath conditions must be regained after the stewing rate of the decision-feedback loop has not been fast enough to keep up with rapid change in the multipath conditions. Data-dependent equalization and ghost-cancellation methods that provide faster convergence than LMS or block-LMS decision-feedback methods are known, but there is difficulty in implementing them in an integrated circuit of reasonable size. Since 60/177,080 was filed, progress has been made with regard to initializing the parameters of the adaptive filter used for echo suppression by data-directed methods, particularly by the xe2x80x9cconstant amplitude modulusxe2x80x9d method. However, it is still desirable to introduce into the A/53 DTV signal a training signal which does not interfere with the operation of DTV signal receivers already in the field and which will rapidly adjust the channel-equalization and ghost-suppression filtering for substantially optimal response.
A/53 specifies the last twelve symbols of the initial data segment of each data field repeat the last twelve symbols of the final data in the preceding data field as a precode signal. This precode signal is specified to implement resumption of trellis coding in the second data segment of each field proceeding from where trellis coding left off processing the data in the preceding data field. This relationship between the initial and second data segments of each data field cannot be disrupted by the introduction of the training signal into the A/53 DTV signal if operation of DTV signal receivers constructed in accordance with A/53 is to be least affected. So, the introduction of the training signal between the initial and second data segments of each data field is undesirable, 60/177,080 points out.
A/53 specifies convolutional interleaving of the data contained in the second through 313th data segments of each data field. These second through 313th data segments of each data field must remain consecutive in time if operation of de-interleavers in DTV signal receivers constructed in accordance with A/53 is to be least affected, 60/177,080 points out.
In accordance with the observations set forth in the preceding two paragraphs, the training signal is best introduced in one or more data segments introduced after the 313th data segment of each data field, in a modification of A/53, 60/177,080 indicates. DTV signal receivers already in the field should have the capability of processing the first complete data field that occurs after a channel change, A/60/177,080 indicates, based on the belief that capability should enable these receivers to accommodate the insertion of additional data segments into each data field. Insertion of two additional data segments per data frame, such as one additional data segment per data field, will reduce data frame rate from 20.66 frames per second to 20.59 frames per second with a 0.32% loss in channel capacity compared to A/53. Insertion of four additional data segments per data frame, such as two additional data segments per data field, will reduce data frame rate from 20.66 frames per second to 20.52 frames per second with a 0.64% loss in channel capacity compared to A/53. Insertion of six additional data segments per data frame, such as three additional data segments per data field, will reduce data frame rate from 20.66 frames per second to 20.46 frames per second with a 0.95% loss in channel capacity compared to A/53.
It is further indicated in 60/177,080 that the twelve-symbol precode signal should no longer repeat the last twelve symbols of the final data segment in the preceding data field, supposing any further data segment containing training signal were introduced at the close of the preceding data field. Instead, the twelve-symbol precode signal should repeat the last twelve symbols of the final data in the preceding data field as it occurs in the 313th data segment of each data field. That is, the twelve-symbol precode signal should repeat the last twelve symbols of the data in the data segment of the preceding data field that precedes the data segment or contiguous data segments in which the training signal occurs. When preparing 60/177,080 for filing before the United States Patent and Trademark Office, A. L. R. Limberg sought preferred types of training signal for inclusion in the additional data segments to be inserted into the A/53 data fields. The training signal should have sufficient energy that the longest delayed ghosts of the training signal have sufficient energy that match filtering using autocorrelation procedures can distinguish these ghosts from interference caused by other signals and by noise. Accordingly, training signals with substantial energy and well-defined autocorrelation responses are a desideratum.
The triple PN63 sequence in the initial data segment of each data field of a broadcast DTV signal as prescribed by A/53 has a well-defined autocorrelation response, but has insufficient energy for detecting longer-delayed post-ghosts with smaller amplitudes. The PN511 sequence in the initial data segment of each data field of a broadcast DTV signal as prescribed by A/53 has substantial energy and a well-defined autocorrelation response. However, no component sequence of the data field synchronizing (DFS) signal or combination of its component sequences has proven in practice to be very satisfactory as a training signal.
One reason is that no portion of the DFS signal is preceded by an information-free interval of sufficient duration that post-ghosts of previous data and data segment synchronizing sequences exhibit insignificant spectral energy during the duration of that portion of the DFS signal to be used as training signal. Also, the A/53 DTV signals do not provide for the generation of an information-free interval of such duration before the training signal by combining information sent at different times, a technique used in de-ghosting NTSC analog television signals. An information-free interval extending over 431 symbol epochs should precede the training signal if it is not to be overlapped by the post-ghosts of previous signals delayed less than forty microseconds or so. When 60/177,080 was filed, it was presumed that post-ghosts delayed more than forty microseconds would not have significant mounts of energy. The post-ghosts of previous signals should be kept from contributing significantly to digitized Johnson noise, in order to preserve the sensitivity of ghost signal detection. It is desirable to extend the duration of this information-free interval by an additional 34 symbol epochs if pre-ghosts advanced by as much as three microseconds are to be detected or by an additional 43 symbol epochs if pre-ghosts advanced by as much as four microseconds are to be detected.
Another reason the PN511 sequence in the initial data segment of each data field of an ATSC broadcast DTV signal is not particularly satisfactory as a training signal is that the PN511 sequence is not repetitive, causing the autocorrelation properties of the PN511 sequence to be compromised. The reader is referred to U.S. Pat. No. 5,065,242 titled xe2x80x9cDEGHOSTING APPARATUS USING PSEUDORANDOM SEQUENCESxe2x80x9d issued Aug. 23, 1994 to Charles Dietrich and Arthur Greenberg. This patent, incorporated herein by reference, points out that the autocorrelation function of a maximal-length pseudorandom noise (PN) sequence has a circular nature. U.S. Pat. No. 5,065,242 describes repetitive PN sequences being inserted as training signal into a prescribed scan line interval of each of the vertical blanking intervals of NTSC analog television signals.
In this specification and the claims appended thereto she phrase xe2x80x9crepetitive pseudo-random noise sequencexe2x80x9d is to be construed as being descriptive of a single continuous sequence, rather than as being descriptive of an intermittently repeated pseudo-random noise sequence. The cycle of a repetitive maximal-length PN sequence is defined in this specification and the claims appended thereto to extend over time until the xe2x80x9crandomxe2x80x9d pattern of binary values thereof begins to repeat. This definition is not at variance with common usage. The cycle of a repetitive maximal-length PN sequence is measured by the time between peaks of the autocorrelation function of the PN sequence.
A many-symbol PN sequence has a reasonably uniform spectrum above zero frequency, so it is suitable for calculating the channel equalization function in the frequency domain particularly if its symbol rate is Nyquist ratexe2x80x94i.e., twice channel bandwidth. U.S. Pat. No. 5,065,242 prescribes channel equalization calculations be implemented using fast Fourier transform (FFT) or discrete Fourier transform (DFT) methods to determine the cepstrum of the transmission/reception channel. The time-domain response of a transmission/reception channel to an impulse, referred to as a xe2x80x9ccepstrumxe2x80x9d, takes the form of a succession of pulses at time intervals indicative of the relative delays of respective multipaths and with amplitudes indicative of the relative amplitudes of those multipaths. U.S. Pat. No. 5,065,242 requires the PN sequences with (2xe2x80x3xe2x88x921) symbols be xe2x80x9craster-mappedxe2x80x9d or stretched in time by to be one symbol epoch longer and thus be of duration equal to 2n symbol epochs, where n is a positive integer greater than one. U.S. Pat. No. 5,065,242 indicates that this stretching, which is done either at the transmitter or the receiver, is done to implement processing of signals by DFT methods to determine the cepstrum.
Such raster-stretching methods are acceptable when dealing with NTSC analog television signals, since the video components of these signals are under-sampled. While the frequency spectrum of the raster-stretched PN sequence signal is reduced compared to a PN sequence with baud-rate symbols, it is still wider than the video components of the NTSC analog television signals. Accordingly, adaptation of the channel-equalization and ghost-suppression filter for optimizing the spectral response of the raster-stretched PN sequence signal over its bandwidth will optimize the spectral response of the video components of the NTSC analog television signals. However, raster-stretching methods are not suitable for A/53 digital television signals, which are sampled at Nyquist rate during their transmission. The adaptation of the channel-equalization and ghost-suppression filter should optimize the spectral response over the bandwidth of a PN sequence with baud-rate symbols, in order that the spectral response over the entire Nyquist bandwidth of the A/53 digital television signals is optimized. This is necessary so that in a receiver for A/53 digital television signals, the baseband digital television signals are Nyquist filtered to limit their bandwidth to one-half the symbol rate before the data slicing procedures used for symbol decoding. In accordance with the Sampling Theorem, such Nyquist filtering eliminates intersymbol interference between symbols occurring at the specified symbol rate. Improper filtering in the Nyquist slope region results in undesirable intersymbol interference.
It is pointed out in 60/177,080 that there is no need for stretching of the PN sequence system function to facilitate searching for ghost positions with a match filter in order to determine the cepstrum. The repetition of a sequence having an autocorrelation function that is circular (i.e., cyclically repeating) in nature permits match filtering using autocorrelation procedures over intervals free from signals other than the training signal and any ghost thereof that could contribute to match filtering response. Only noise contributions to match filter response need be considered when determining the match filter capability to detect weak-energy ghosts of the training signal.
Computation of the cepstrum by DFT methods can be done by extending the PN sequence composed of equal-value positive and negative samples by enough zero-valued null samples to have 2p samples, where p is a positive integer greater than one and where 2p is more than twice the number of symbols in the PN sequence, without spectral content in relative amplitude terms being appreciably affected. Indeed, DFT used in such calculations presumes the signal segment being transformed is flanked by zero-valued samples anyway. Accordingly, as was pointed out in 60/177,080, repetitive PN sequences with symbols at customary baud rates are suitable training signals for inclusion in digital television signals, especially if suitable precautions are taken in positioning these repetitive PN sequences in the DTV signals.
Since 60/177,080 was filed, pre-echoes leading the strongest multipath signal component by as much as thirty microseconds have been reported as being observed during the field testing of DTV receivers using indoor antennas. Furthermore, post-echoes lagging the strongest multipath signal component by about sixty microseconds were reported to have been observed in the New York city area, which post-echoes are caused by reflections from the suspension bridge across the Verrazano Straits. DTV receivers for accommodating ranges of significant echo energy that extend over as much as 90 microseconds have been proposed by A. L. R. Limberg, modifying the teachings in 60/177,080 so as to use repetitive-PN1023 sequences as training signals.
In an aspect of the invention training signals for adaptive channel-equalization and ghost-suppression filtering in receivers for broadcast digital television signals are inserted in those broadcast DTV signals, which training signals consist of similar symbol sequences, each of which symbol sequences reposes within plural consecutive data segments and incorporates each data segment synchronizing (DSS) signal therebetween. In a further aspect of this invention, each training signal is preceded in the broadcast TV signal by a respective information-free interval of a prescribed number of symbol epochs. In a further aspect of this invention, each training signal is a repetitive pseudorandom-noise (PN) sequence continuing more than six hundred symbol epochs, to facilitate receivers for broadcast TV signals detecting the relative positions and amplitudes of the principal signal and its post-ghosts delayed as much as forty microseconds therefrom. In a still further aspect of this invention, the inclusion of each training signal in the broadcast TV signal is accommodated by adding at least one additional data segment near the conclusion of each data field to increase the number of data segments in each data field to more than 313.
Other aspects of the invention are embodied in receivers for receiving broadcast DTV signals and for utilizing the training signals specified in the foregoing paragraph. Certain digital television receivers embodying the invention use the repetitive PN sequence as the basis for calculating the initial values of the weighting coefficients of the adaptive filtering used for channel equalization and ghost suppression. Thereafter, the weighting coefficients are updated by decision-feedback techniques relying on received data or by repetition of the method used to initialize those weighting coefficients.
Certain digital television receivers embodying the invention include novel pedestal-suppression filters cascaded before match filters for PN sequences. These pedestal-suppression filters separate the repetitive PN sequences used as training signals from accompanying direct component generated during the synchronous detection of broadcast TV signals. The repetitive PN sequences separated in the responses of the pedestal-suppression filters supply input signals for the match filters, the responses of which match filters are used for generating cepstrum signals that locate the timing and amplitudes of ghosts relative to the principal received signals.